What button is this? (Android App Menubar)

Does anyone know what this button does? Long pressing doesn’t seem provide an explanatory popup. I think it has something to do with saving/syncing, but saving & syncing seem to happen regardless of whether I press that button.

Nevermind, it seems to be a multi-select button according to this reddit post: https://www.reddit.com/r/dynalist/comments/nfy8cm/can_anyone_tell_me_what_does_this_button_do/